Orthodontic appliances, and in particular orthodontic brackets, are made of various materials including ceramics (e.g., mono- or poly-crystalline alumina), plastics (e.g., polycarbonate), and metal. Non-metallic brackets are most commonly chemically bonded to the tooth enamel. In order to enhance the chemical bonding of a bracket to a tooth, it is known to provide the bracket with a chemically active bonding surface or bonding base which interacts with the chemical adhesive to form a suitably strong bond to a tooth. For example, U.S. Pat. Nos. 4,826,430 and 4,948,366 describe the preparation of ceramic brackets having chemically active bonding surfaces, and U.S. Pat. No. 4,673,354 discloses brackets made of porcelain, metal alloys and resin composites which have a chemically active layer.
A significant limitation of brackets which have a chemically active bonding base or surface is their sensitivity to contamination. To ensure the maximum desired bond strength, it is imperative that contact of the bonding surface with foreign matter be eliminated. Any contamination of the chemically active bonding surface causes physical hinderance of the molecular functionality thereof, which reduces chemical coupling (bond strength) by an amount generally corresponding or proportional to the degree of contamination. Since it is common to bulk package brackets with a chemically active surface, there is a significant risk of contamination as the brackets tumble about and contact one another during transportation and handling. Further potential for contamination is possible from finger contact as the brackets are removed from packaging and manipulated during chair side set-up by the clinician. Contamination of the chemically active bonding surface causes the contaminated regions to become effectively chemically "dead". It is believed that contamination thus deleteriously affects the integrity of the bond which is ultimately achieved, potentially resulting in clinically unacceptable bonding of brackets to teeth. Poor bonding may result in failure during the course of the orthodontic treatment, thereby necessitating return visits to the orthodontist for replacement/reattachment of brackets.